1. Field of the Invention
The present invention relates to a programmable controller (or programmable ladder controller, hereinunder referred to as "PC") for automatically controlling various equipments in accordance with a sequence program.
2. Description of the Related Art
PC's are used for controlling various pieces of equipment such as a transfer machine and a PC having more functions and easy to use has recently been demanded for realizing factory automation.
As shown in FIG. 1, a conventional PC 10 includes a memory 12 for storing a sequence program and a control processor 14 for controlling a piece of equipment in accordance with the sequence program. The PC 10 further includes I/O portion (not shown) connected to an external piece of equipment, data memory (not shown), system memory (not shown), etc. FIG. 2 shows what is called a relay ladder for showing the order and the conditions of control with respect to the equipment being controlled. Equipment being controlled includes a virtual equipment (dummy equipment) which exists only in a program.
A relay symbol 16 in the relay ladder diagram represents a piece of equipment being controlled. An address 18 shown in the vicinity of the relay symbol 16 is a local I/O address which is only effective in the PC 10. The I/O address is expressed by an octal digit, a hexadecimal digit or the like and individually allotted to each piece of equipment.
The order and the conditions of control shown in the relay ladder diagram are programmed by a programming language such as a logic symbol language and a relay symbol language and stored in the memory 12 as a sequence program. To state this more concretely, in the sequence program, a command and an I/O address to each piece of equipment are entered, as shown in FIG. 1. The sequence program memory 12 therefore includes a region 12a for storing a command and a region 12b for storing an address.
Such a conventional PC, however, is disadvantageous in that it is very difficult to check or correct a program.
When a program is checked or corrected, a programming tool is connected to the PC. In this case, the I/O address in the program which is displayed on the programming tool is expressed merely by a numeral (numeral such as an octal or hexadecimal digit). It is therefore difficult for the operator to judge which equipment the I/O address indicates.
Accordingly, at the time of creating, checking or correcting a program, the operator must constantly confirm the I/O address with reference to the relay ladder diagram, which is the basis of a program. According to this programming method, the operability is bad and an inputting error is easily caused.
Especially, in a system in which a plurality of PC's are dispersively arranged, the above-described problems are more prominent. Since the same I/O address exists in each PC, it is difficult for the operator to identify the I/O address allotted to the equipment at the time of creating a program not to mention at the time of checking and correcting the program.
Japanese Patent Publication No. 46683/1976 discloses a system in which a plurality of PC's are dispersively arranged for a parallel processing. In this system, addresses different from each other are allotted to the respective pieces of equipment. This system, however, has a complicated rule for allotting the addresses, so that it is not easy to create a program and correct it. It is also not easy to judge to which equipment an address corresponds.